1. Field of the Invention
The present invention relates to ultrasonic imaging systems, and more particularly, to ultrasonic imaging systems and methods which create the effect of imaging with a filled linear array while making use of a reduced or minimum number of linear array elements or processing channels.
2. Description of the Prior Art
In the imaging of coherent scenes composed of distributions of reflectors, active (transmit/receive) imaging arrays have been used in the prior art. In such imaging systems, the imaging array is often composed of linear arrays of elements which can function as both transmitters and receivers for, e.g., far-field, active imaging using narrowband radiation. As known to those skilled in the art, an image may be obtained with such a system by forming transmit and receiving beams and scanning them together across the scene Such beams are typically characterized by their beam patterns, where the beam pattern of the array is defined as its complex gain as a function of direction of arrival of incident radiation. Examples of imaging systems making use of such arrays may be found in the fields of medical ultrasound and underwater acoustic imaging.
As known by those skilled in the art, ultrasonic imaging is a technique used to form an image of the interior of a solid, opaque object by use of high-frequency, low amplitude mechanical vibrations (ultrasound). As just noted, this technique has applications in medical and underwater acoustic imaging. In medical imaging, images of small parts of the body (valves of the heart, for example) are formed and displayed. Such applications put a premium on high image resolution and overall image quality. For such reasons, it has been proposed to use phased arrays of transducer elements and signal processing techniques in order to obtain higher quality images from a given ultrasound transducer and instrument than would ordinarily be obtainable from that equipment. However, to date, sufficient techniques have not been developed.
Many prior art ultrasonic imaging instruments make use of single-element, mechanically scanned transducers, where the transducer element is a source of ultrasound which may also function as a receiver. In a mechanically scanned system, this element is pointed in a direction of interest by mechanical means and then is excited by a high-voltage pulse which causes it to emit ultrasound. Because the aperture is typically large compared to the wavelength of the emitted sound, the strongly insonified region is limited to a small volume of space directly in front of the transducer. This region is known as the afore-mentioned "beam". The echoes from the transmission are, in turn, sensed by the transducer element and recorded. Then, when the beam has been scanned over all the directions of interest, the recorded echoes may be combined to create an image which is displayed on a display device such as a CRT. In a phased array ultrasound instrument, multiple elements of this type are used. In many such systems, these elements are deployed side-by-side in a line to form a linear array, while in other systems the elements are made in the shape of continuous rings of annuli of varying radius.
As noted above, a beam may be formed in an array imaging system by transmitting (or receiving) with all of the elements at once. When a linear phased array is used, scanning of the beam is accomplished by varying time delays imposed on the measured echoes of the different elements, while the annular phased array is mechanically scanned and is usually focused by imposition of the delays on the transducer elements In the case of the linear phased array, the standard method of image formation is thus beam forming and scanning.
There are several known applications of phased arrays in medical ultrasound at the present time. In abdominal imaging (pre-natal, for example), large phased arrays are sometimes used, although the most common mode of operation is that of the large, unsteered linear array which translates the path of the sound through the body by subarray selection. Phased arrays are also used in cardiac imaging, but they are restricted in their physical size by the requirement that imaging be done through the spaces between the ribs. Phased arrays are also used in most color flow mappers, which image moving blood using a Doppler shift to project motion, because, unlike moving transducers, phased arrays can illuminate the same volume over and over again. Annular arrays are also widely used; however, their primary value lies in the fact that they can be dynamically focused when receiving an ultrasound echo.
Active imaging systems of the type to which the present invention is directed may be characterized by their point spread functions (PSF). The PSF is simply the image produced by the system for a point target or point reflector, and since any linear imaging system can be characterized by its response to a point reflector, the PSF is the key determinant of image quality. In other words, the quality of the final image is determined by the PSF. Because of this feature of active imaging systems, a method is desired for synthesizing a desired PSF in an image obtained with a given array used for active imaging. The desired PSF must belong to the set of realizable PSFs associated with the aperture, for the set of realizable PSFs for a given aperture may be larger than the set of PSFs which can be obtained by the standard method of transmit/receive beam forming and scanning. As will be described herein, images with these PSFs can be synthesized by the use of image addition, which is referred to herein as image synthesis or aperture synthesis. Such synthetic aperture active imaging systems using arrays have been a topic of investigation for some time, and various schemes have been proposed which are motivated by the desire to create the effect of a large array without using a large number of array elements. However, most of the schemes in the prior art, such as those described by W. H. Wells in "Acoustical Imaging With Linear Transducer Arrays," Acoustical Holography, Vol. 2, pp. 87-103 (1969), and by P. N. Keating et al. in "Holographic Aperture Synthesis Via a Transmitter Array," Acoustic Holography, Vol. 6, pp. 485-523 (1975), were devised on an ad hoc basis and do not allow for specification of a desired PSF. Additionally, such schemes typically deal with planar arrays, and the applicability of the idea of redundancy to linear active imaging arrays has not been explicitly recognized and exploited in the prior art.
Accordingly, the present invention has been designed to specifically apply the idea of redundancy to linear active imaging arrays so as to allow for improved imaging efficiency, and as will be described herein, this application has led to a significant advancement in the acoustical imaging art.